Colorant dispersions require low viscosity, which may be accomplished through certain dispersants. By lowering the viscosity, more colorant may be added, affording very concentrated dispersions which can be satisfactorily handled and dispersed. Dispersions with high colorant content provide a number of advantages to for example, a printing ink manufacturer seeking to employ the dispersion, such as greater formulating latitude which permits the production of a greater variety of printing inks, cost reduction and lower inventory volumes.
The upper limit for the colorant concentration in a dispersion is determined by the resulting viscosity of the dispersion. As the colorant concentration increases linearly, the viscosity of the dispersion increases exponentially. If the viscosity becomes too high, the colorant concentrate can no longer be dispersed satisfactorily or easily handled during the ink manufacturing process. In addition, as the colorant concentration and resultant viscosity increases, the ink system experiences increasing heat build up due to frictional forces, during the dispersion process. This will be detrimental to the quality of ink as the increase in temperature will cause the degradation of certain colorants. Additionally, without proper control, lithography problems such as over-emulsification occur, which is when the ink takes up excessive fountain solution causing poor ink transfer, blanket piling, high dot gain, and misting; scumming, which is the inability of fountain solution to keep the non-image area of the printing plate clean and which is not easily washed from the plate; and feedback, which is the contamination of fountain solution during the printing process resulting in poor final print quality.
U.S. Pat. Nos. 4,859,247, 4,224,212, 4,057,436 and 3,996,059 describe oil-soluble viscosity-reducing salts or cationic dispersant additive materials. Incorporation of these additives allow for an increase in pigment concentration in the respective dispersions but their “surfactant-like” properties create a low interfacial tension, a very large relative interfacial tension drop in the oil-water ink system and lithographic printing problems (e.g. over-emulsification, scumming, or feedback). U.S. Pat. No. 6,540,825 describes a pigment dispersion which is the reaction product of an alkanolamine and a hydrocarbyl succinic anhydride compound to be used in aqueous and/or non-aqueous media. However, the dispersion suffers from low pigment strength and has less than 45 wt. % pigment in final dispersion. U.S. Pat. No. 6,290,768 discloses a pigment concentrate made up of 50-90 wt. % pigment reacted with large amounts of natural resins or natural resin acids, monocyclic or polycyclic phenols, and small amounts of aldehydes or aldehyde acetals, groups Ia and IIa metal compounds, alpha, beta-ethylenically unsaturated carboxylic acids or their anhydrides. While this pigment dispersion preparation has good flow, it has high viscosity which creates ease of handling problems. U.S. Pat. No. 5,935,315 discloses low viscosity aqueous pigment dispersions free of organic solvents that contain a fatty amidoalkoxylate dispersant. The C9-C19 polymer contains a C2-C3 hydrocarbon moiety or alkyleneoxy group. U.S. Pat. No. 5,882,393 discloses a dispersing agent or the salt of polyisocyanates reacted to a heterocyclic ring and a C2-C5 acrylate or epoxide moieties. Dispersants that contain such salts exhibit poor lithographic printing performance. U.S. Pat. No. 5,645,636 discloses a 5-50 wt. % pigment concentrated base and a dispersant of arylpararosanillnesulfonic acids. The dispersant is the condensation product of aliphatic amines or ammonium compounds and polyesters of hydroxyl fatty acids. U.S. Pat. No. 5,688,312 discloses a hot melt ink composition containing polymeric imides or bisimides made by reacting C2-C30 polyalkyleneoxide with a derivative of phthalic anhydrides or succinic anhydrides. U.S. Pat. No. 5,336,314 and WO 02/34840 A1 both describe dispersants, however neither resulting dispersing vehicles can be used in conventional flush systems. U.S. Pat. No. 5,336,314 discloses reacting phthalic anhydride with a polyester polymer. WO 02/34840 A1 discloses a phthalic acid imide with a C30 maximum chain polyalkyl length for use in water-based systems. U.S. Pat. No. 5,429,841 discloses an ink composition containing 0.1 to 15 wt. % of polyoxyalkylene substituted colorants. U.S. Pat. No. 4,859,247 discloses an oil-soluble viscosity-reducing additive made by combining a long chain aliphatic amine having a molecular weight between 80 and 340 and a polybasic organic acid having a molecular weight between 50 and 250 in the form of salt. Incorporation of the additive allows increase pigment concentration in dispersion of up to 65 wt. %. However, these types of salts create significant problems in lithography such as scumming and feedback. U.S. Pat. No. 4,765,841 describes a preparation of organic pigment dispersion in a non-aqueous medium directly from aqueous slurry of the pigment. Dispersants are selected from resins, alkyds and mixed amide/salt-derivatives of polyesters derived from hydroxycarboxylic acids. These dispersants cause a substantial decrease in interfacial tension and cause the aforementioned scumming, feedback and over-emulsification problems. U.S. Pat. No. 4,468,255 discloses that diazo and azo linkage Schiffs bases reacted with polyoxyalkane polymeric chains can be used as a rheological modifier for pigment dispersions. U.S. Pat. No. 4,461,647 discloses a non-aqueous pigment dispersant comprising a diazo compound that contains a single substituted ammonium-acid salt group. In general, salts create significant problems in lithography such as scumming and feedback. U.S. Pat. No. 4,391,648 discloses a solid particulate granular pigment composition which is readily dispersible in oleoresinous vehicle systems made of from 25 to 95 wt. % of at least one pigment; from 1 to 15 wt. % of a cationic surfactant or blend of surfactants; and from 5 to 75 wt. % of a water-soluble, non-crystalline, friable, thermoplastic polyester resin having a hydroxyl value of 75-150 mg. The surfactants placed in the dispersion cause scumming and feedback. U.S. Pat. Nos. 4,340,341 and 4,309,320 both describe a preparation of an organic pigment dispersion in a non-aqueous medium from an aqueous slurry of the pigment by incorporating an additive selected from the class of sulfonated sperm oil and sulfated sperm oil. However, the pigment load is low at about 35-40 wt. % and 10 wt. % respectively. U.S. Pat. No. 4,057,436 discloses a pigment dispersion with between 5-70 wt. % pigment using a quaternary ammonium salt of a monomeric colored acid wherein there are from 19 to 60 atoms contained in the 4 chains attached to the N-atom of the quaternary ammonium ion. Quaternary ammonium compounds tend to cause the aforementioned lithographic problems and low interfacial tension. U.S. Pat. No. 4,028,128 describes a pigment dispersion which contains a carboxy ester between 5-200 wt. % off pigment in organic liquids useful for paints, lacquers and gravure inks. However, pigment concentrations are low. U.S. Pat. No. 3,966,059 discloses a dispersion of pigment content from 5-50 wt. % and with a dispersion agent with the formula Y—CO—B—A—R. The dispersions greatly suppress interfacial tension and result in poor lithographic performance. U.S. Pat. No. 6,037,414 describes a graft polymer having acrylic polymer backbone and pending from this backbone polyester side chains, cyclic imides, and quaternary ammonium groups. These molecules range in molecular weight from 8,000-50,000. The dispersants described are well known ammonium groups which cause problems in lithographic performance such as scumming and feedback during the printing process.
As indicated hereinabove, the prior art has disclosed colorant dispersions with more than 45 wt. % pigment and viscosities less than 150 Pa·s, however, when used in lithography, said dispersions have been known to cause problems (e.g. pigment over-emulsification, scumming, or feedback). Thus, there exists a need for a dispersant that lowers the viscosity of highly concentrated non-polar colorant dispersions, while preserving a high colorant concentration.
Further, there exists a need for a highly colorant dispersion which has the properties cited above while only employing a relatively small amount of dispersant.
Finally, there exists a need for a dispersant which is dust-free and therefore easy to dispense.